The invention relates generally to the trimming of monolithic integrated circuits wherein the integrated circuit (IC) is first fabricated in wafer form to create a plurality of circuits on a common substrate. The wafer is then subjected to testing wherein the wafer is located in a step and repeat machine. The machine has the capability of moving the wafer in two dimensions, each of which can be precisely adjusted to the spacing between adjacent circuits. The machine includes an array of test probes that can be adjusted so that the probes coincide with the bonding pads on the circuit chips located on the wafer. In operation, the machine advances the probe assembly to contact a circuit. Then the assembly is lifted out of contact, the wafer indexed or stepped one circuit position, and the probes brought back into contact. The stepping sequence is repeated so that each circuit chip can be contacted individually. During the contact interval, the probes can be supplied with power supply potentials, signal potentials, and measurement instruments. Thus, during probe contact a circuit can be activated so as to be electrically operative and its performance measured. The step and repeat operation can be programmed so that each circuit on the wafer can be sequentially tested on an automatic basis and the circuit performance recorded.
If desired, the circuit can also be trimmed during the probing in order to modify the circuit performance to bring it within a predetermined specification. One such trimming method employs the so called "zener zapping". It has been found that when a zener diode is pulsed it can be made to short out and in effect revert to act as an ohmic low resistance connection. To use this characteristic, the IC to be trimmed is provided with a multielement resistor in which each element is shunted by a zener diode. Each diode is provided with metal connections that lead to bonding pads that can be probe contacted. During wafer test the probes associated with a particular diode can be supplied with an electrical pulse that will cause that diode to be "zapped" or shorted out. To trim the circuit, as many diodes are shorted out as required to bring the circuit performance to a desired level. The only circuit requirement associated with this form of trimming is that the operating circuit does not produce a potential across the trimmable resistor section that is large enough in normal operation to cause the diode to conduct. Accordingly, the trimmable resistors are normally associated with circuit portions operating at low potentials.
While the foregoing discussion refers to resistors being trimmed, other circuit components can be employed. For example, the trimmable elements can be capacitors. A plurality of series connected capacitors are arrayed with each one having a parallel connected zener diode. Whereas in the resistor system, the unzapped diodes result in maximum resistance, in the capacitor system the unzapped diodes would result in the lowest capacitance.
One of the major disadvantages of zener tap trimming is that a large number of pads are required to implement the system and this requires substantial chip area that is not in active use. The number of pads required is N=n+1, where N is the number of pads and n is the number of diodes.